Cutting tools are used for removing material from a workpiece to manufacture therefrom of desired final element. There exists in common practice a great variety of operations for the removal of material, for each of which, a specific tool is designed. Examples of such operations are drilling, milling, turning, boring etc.
The majority of cutting tools are formed with a sharp cutting edge adapted to come in contact with the workpiece, within a cutting zone, to remove material therefrom in the form of a chip, in a process referred to as chipping. During such process there is usually provided a linear displacement of the cutting tool relative to the workpiece, referred to as ‘feed’ and either rotation of the cutting tool with respect to the workpiece, such as e.g. in milling and drilling, or rotation of the workpiece with respect to the cutting tool, such as e.g. in turning.
Chips must be evacuated continuously during chipping and any congestion may rapidly lead to high loads, overheating and consequent break-down and failure of the cutting tool.
The cutting edge of a cutting tool is generally defined as an intersection line between a rake surface and a relief surface.
The rake surface is adapted to come in contact with the removed chip and, as such, its geometry influences the length and geometry of the removed chip, and more importantly, the manner of evacuation of the chip from the cutting zone.
The relief surface is generally designed so as not to come in contact with the portion of the workpiece from which the chip has been removed. Depending on the angle between the relief surface and the rake surface, the cutting tool is positioned such as to avoid contact between the relief surface and the workpiece.
Normally, chipping may be divided into two general types—rough chipping type and fine chipping. During rough chipping, an amount M of material is removed from the workpiece, providing it with the general shape approximating that of the final desired element. During fine chipping, the workpiece is brought to the final shape by fine and precise removal of material of an amount m. Clearly, the amount of removed material M is normally much greater than that of the removed material m, and it constitutes the majority of the overall removed material. Thus, it would be understood that the majority of time spent on manufacturing a desired element from a workpiece is dedicated to rough chipping.
The amount of material to be removed per time unit by a given cutting tool from a given workpiece is usually controlled by the feed and speed of rotation of the cutting tool relative to the workpiece. The thickness of the chip removed from the workpiece is determined by the speed of rotation VR as well as the feed F. Rotation speed VR is, in turn, determined by the power provided by a motor responsible for the rotation.
Loads exerted on the cutting tool during chipping may be very high and they depend drastically on the feed F and rotation speed and VR, as well as the materials from which the cutting tool and workpiece are made. The loads exerted on the cutting tool may be so high as to inflict damage on the thereon and rendering it useless. On top of this, friction of the cutting tool with the workpiece cause an increase in the temperature of the workpiece and require extensive cooling, usually in the form of a cooling liquid. Thus, the feed F and rotation speed VR are limited, and so is, as a consequence, the power provided by the motor. Thus, feed and rotation speed F and VR usually range between 1-2.5 m/s and 500-1000 rpm respectively.
It would also be appreciated from the above that, in general, the loads on the cutting tools are much greater during the rough chipping than during fine chipping.